Add union_with to sorted set. (#4901)

* Add union_with to sorted set.

Author: wu-hui

Firestore/core/src/firebase/firestore/core/sync_engine.cc

@@ -402,10 +402,8 @@ DocumentKeySet SyncEngine::GetRemoteKeys(TargetId target_id) const {
     }
 
     for (const auto& query : queries_by_target_.at(target_id)) {
-      for (const auto& key :
-           query_views_by_query_.at(query)->view().synced_documents()) {
-        keys = keys.insert(key);
-      }
+      keys = keys.union_with(
+          query_views_by_query_.at(query)->view().synced_documents());
     }
     return keys;
   }

Firestore/core/src/firebase/firestore/immutable/sorted_set.h

@@ -74,6 +74,23 @@ class SortedSet : public SortedContainer {
     return SortedSet{map_.insert(key, {})};
   }
 
+  ABSL_MUST_USE_RESULT SortedSet union_with(const SortedSet& other) const {
+    const SortedSet* result_ptr = this;
+    const SortedSet* other_ptr = &other;
+
+    // Make sure `result_ptr` always points to the larger one of the two sets.
+    if (result_ptr->size() < other_ptr->size()) {
+      result_ptr = other_ptr;
+      other_ptr = this;
+    }
+
+    auto result = *result_ptr;
+    for (const auto& k : *other_ptr) {
+      result = result.insert(k);
+    }
+    return result;
+  }
+
   ABSL_MUST_USE_RESULT SortedSet erase(const K& key) const {
     return SortedSet{map_.erase(key)};
   }